Abstract

Few long-term fertilization experiments have been performed in forests, even though the effects of nitrogen (N) addition on soil microbial biomass are a cause for concern. Our objective was to examine the effects of repeated fertilization for 36 years on soil microbial biomass in two forest stands. We measured soil chemical properties and microbial biomass carbon (C) and N in soils in fertilized and non-fertilized plots in a birch stand (Betula maximowicziana Regel) and a fir stand (Abies sachalinensis Fr. Schmidt). We also performed lime amendments and a 21-day laboratory incubation, and measured microbial biomass to clarify the effects of acidification due to fertilization. Soil pH was significantly lower in fertilized plots in both stands, and soil microbial biomass C and N were lower (significantly so in the fir stand) in the fertilized plots after 36 years of repeated fertilization. In the laboratory incubation, lime amendment did not significantly affect the microbial biomass C, N, or C:N ratio, despite an increase of about 1 unit in soil pH. Our results therefore indicate that factors other than soil pH also have important effects on soil microbial biomass in repeatedly fertilized forest stands.

Notes

Acknowledgements

We are grateful to T. Hashimoto of FFPRI for his support and to R. Takeuchi, S. Katsui, M. Nemoto, and E. Ihara (FFPRI) for assisting with the laboratory work. We also thank A. Oda, M. Komatsu, and Y. Mizoguchi (FFPRI) for their valuable suggestions regards our research. This study was supported by a grant from the Ministry of Education, Culture, Sports, Science and Technology of Japan (no. JP26450218) and by the FFPRI Encouragement Model in Support of Researchers with Family Responsibilities.

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